A shell tore through a United Nations school in the Gaza Strip, killing 20 people who had taken shelter there, amid dozens of assaults the Israeli military carried out across the Hamas-run territory overnight.

UN official Adnan Abu Hasna told Israel’s Army Radio the building in Jabaliya refugee camp in northern Gaza was struck by Israeli tank or artillery fire. The Israeli army said it was checking the report.

The deaths pushed the number of Palestinians killed in three weeks of fighting above 1,200, Ashraf al-Qedra, spokesman for the Health Ministry in Gaza, said. Fifty-six people have died on the Israeli side, all but three of them soldiers.

President Barack Obama and UN chief Ban Ki-moon are among world leaders calling for an immediate cessation of the third major military showdown in Hamas-ruled Gaza in less than six years. A bid by the U.S., UN and Egypt to broker a truce last week failed to win agreement on anything deeper than an hours-long humanitarian halt over the weekend.

U.S. Secretary of State John Kerry, who spearheaded those talks, told reporters yesterday he’s still in regular contact with Israeli Prime Minister Benjamin Netanyahu as truce efforts continue. Netanyahu on July 28 told his country to brace for an extended campaign.

Rockets

The third major military showdown between the sides in less than six years has claimed the lives of more than 1,100 Palestinians, most of them civilians, and 55 Israelis during three weeks of fighting, including 10 soldiers killed yesterday.

Obstacles to a more lasting deal include Israel’s demand for the demilitarization of Gaza and Hamas’s insistence on an end to the blockade of the territory Israel initiated in 2006. Israel, like the U.S. and European Union, labels Hamas a terrorist organization, and says it uses civilians as human shields.

Hamas’s military chief ruled out a truce in Gaza until Israel lifts its blockade of the Palestinian territory, as the military stepped up its bombardment. Hamas’s armed wing, the al-Qassam Brigades, can withstand the Israeli offensive, Mohammed Deif said, in recorded comments shown late yesterday on the group’s Al-Aqsa television channel.

“The balances of power now are different and the gun battles showed that the Palestinian armed resistance is stronger and more powerful than the army of the enemy,” Deif said.

Rockets fired from Gaza struck southern Israel early today, causing no injuries or damage. During the latest conflict, Hamas has fired about 2,700 rockets at Israel and Israel has hit 4,100 “terror sites,” according to the army.